diff --git a/packages/griffelib/src/griffe/_internal/expressions.py b/packages/griffelib/src/griffe/_internal/expressions.py
index 26a97d15..b3f833f7 100644
--- a/packages/griffelib/src/griffe/_internal/expressions.py
+++ b/packages/griffelib/src/griffe/_internal/expressions.py
@@ -164,7 +164,13 @@ class Expr:
     """Base class for expressions."""
 
     def __str__(self) -> str:
-        return "".join(elem if isinstance(elem, str) else elem.name for elem in self.iterate(flat=True))  # ty:ignore[unresolved-attribute]
+        rendered = "".join(elem if isinstance(elem, str) else elem.name for elem in self.iterate(flat=True))  # ty:ignore[unresolved-attribute]
+        # A top-level named expression (walrus) is invalid Python without surrounding
+        # parentheses, e.g. as an attribute value or a parameter default. Nested occurrences
+        # are parenthesized by their enclosing context, but the top level has none.
+        if isinstance(self, ExprNamedExpr):
+            return f"({rendered})"
+        return rendered
 
     def __iter__(self) -> Iterator[str | Expr]:
         """Iterate on the expression syntax and elements."""
diff --git a/packages/griffelib/tests/test_expressions.py b/packages/griffelib/tests/test_expressions.py
index 2f0d3bf5..6fba48db 100644
--- a/packages/griffelib/tests/test_expressions.py
+++ b/packages/griffelib/tests/test_expressions.py
@@ -142,6 +142,7 @@ def test_await_expression(source: str, expected: str) -> None:
     "f'{x:>{width}}'",
 ]
 _expression_contexts = [
+    "%s",
     "f(%s)",
     "f(%s, 1)",
     "f(a=%s)",
@@ -199,6 +200,18 @@ def test_length_one_tuple_as_string() -> None:
         assert str(module["x"].value) == "('a',)"
 
 
+def test_top_level_named_expression_keeps_parentheses() -> None:
+    """A top-level named expression (walrus) must keep its surrounding parentheses.
+
+    Without them, the rendered string is invalid Python as an attribute value
+    or a parameter default (e.g. `x = n := 1` is a syntax error).
+    """
+    code = "x = (n := 1)\ndef f(a=(b := compute())): ...\n"
+    with temporary_visited_module(code) as module:
+        assert str(module["x"].value) == "(n := 1)"
+        assert str(module["f"].parameters["a"].default) == "(b := compute())"
+
+
 @pytest.mark.parametrize(
     ("annotation", "modernized"),
     [